Reducing Power Consumption of the Issue Logic

1. Reducing Power Consumption of the Issue Logic Daniele Folegnani and Antonio González Universitat Politècnica de Catalunya

2. MOTIVATION • Power consumption • High performance microarchitecture • Cooling systems • Reliability • Embedded systems • Battery life

3. OUTLINE • Power Consumption in Superscalar Processors • IPC-based Instruction Queue Resize • Results • Conclusions

4. Power Evaluation Methodology Dynamic Power Estimator[Cai,Lim MICRO32] • Architectural design partition • Architectural block fits a circuit block • Power consumption evalutation at block level • Power density of blocks (SPICE, input sets, technology and circuit styles definition) • Blocks and sub-blocks activity (execution-driven) • Area (feedback from VLSI design)

5. The Power Model • 0.18 microm CMOS • 5 Types of logic (static, dynamic, SRAM, clock, PLA) • 32 Blocks and area associated • Custom design • Power densities ( APD, IPD )

6. EXPERIMENTAL FRAMEWORK • 4 instr. fetch, issue and commit • 128 entries instruction queue size • I-Cache 128Kbytes, direct mapped, 32 byte line, 1 cycle hit, 3 cycle miss • D-Cache 128Kbytes, 4 way set ass, 32 byte line, 1 cycle hit, 3 cycle miss • UL2-Cache,1024Kbytes, 4 way set ass, 64 byte line, 3 cycle hit • Combined predictor of 1K entries with Gshare with 1K 2-bit counters, • 8 bit global history and bimodal pred. of 2K entries with 2-bit counters • 4 intALU, 4fpALU, 1int mul/div, 1 fp mul/div • Out of order issue, oldest ready first selection policy

7. Power Consumption in Superscalar Processors

8. ANALYSIS • Power Analysis • IQ + ROB = 53% of total consumption • Almost independent to instruction mix • Trends in Superscalar • Increasing • IW • entries in the window IQ Power contribution may grow in the future

9. ANALYSIS • Considering • Periods of execution with low parallelism • Some parts of the IQ has negligible impact on total IPC • Periods of execution with high parallelism • Few parts of IQ can satisfy the issue width

10. ISSUE IN THE IQ

11. ISSUE IN THE IQ

12. COMMIT IN THE IQ

13. COMMIT IN THE IQ

14. IPC-based Instruction Queue Resize • IQ Resize • Based on IPC contribution • Avoid wake-up on disabled parts IQ has a circular FIFO without collapsing

15. IPC-based Instruction Queue Resize • IQ Resize • IQ physically divided in 16 parts of 8 entries • Add the limit pointer, updated as the head pointer • At resize time, move the limit of one part • If limit reach the tail, stop to insert new instructions

16. Heuristics • Heuristic to reduce size • Statistic of committed instructions in youngest part every quantum time => add a bit in each ROB entry • Threshold based resize decision • No size limit to disable • Heuristic to grow size • Grow one portion every 5 quantum time • The threshold based scheme will decide the correctness • the next quantum time

17. Results

18. Conclusions • IQ is a the critical point for power consumption in superscalar processors • Dynamically adapting the IQ size based on IPC contribution can save about 15% of total power with negligible impact on performance

19. Q & A ?